ZYMO-EXCITATORS OR KINASES 217 



by Langley in the case of pepsin, and the precursor of the enzyme 

 was termed pepsinogen. The method of separating pepsinogen 

 and pepsin, by their varying resistance to alkali, which attacks 

 pepsin with much greater rapidity than it does pepsinogen, was 

 later given by Langley and Edkins. Trypsinogen, the zymogen 

 of trypsin, was later shown to exist by Heidenhain, and since then 

 the existence of a preferment has been shown for most of the 

 enzymes. These zymogens, as has been stated, are inactive while 

 in the cell and exist in granular form visible under the microscope; 

 they are converted into the active form, either at the time of 

 secretion or later, on coming in contact with certain substances 

 which have been termed zymo-excitators, or, in certain cases, 

 kinases. 



This action is possessed by all dilute acids, and it is probably 

 in this way that pepsinogen and prochymosin, the zymogens 

 respectively of pepsin and of rennin or chymosin, are activated in 

 the stomach. 



In the case of the trypsinogen of the pancreatic juice it has 

 been shown by Pawlow and Chepowalnikoff that the activation 

 takes place by means of a substance secreted by the intestinal 

 mucosa. It was found that while the secretion from a pancreatic 

 fistula had scarcely any action upon protein, the addition of a 

 small quantity of succus entericus caused it rapidly to become 

 very active. Such action took place only upon the trypsin and not 

 upon the other preferments of the pancreatic juice, but an increase 

 in the activity of the lipoclastic enzyme occurred on the addition 

 of bile. 



The substance which so behaved as a zymo-excitator or kinase 

 to trypsinogen was called enterokinase by Pawlow, and has since 

 been the subject of much investigation and discussion as to whether 

 it is itself a true ferment, a " ferment of ferment," as it was styled 

 by its discoverer, who regards it as a ferment on account of the 

 small quantity necessary to activate a much larger amount of 

 trypsinogen, and the fact that it is destroyed, although slowly, 

 at the usual temperature of destruction of enzymes (65 C.). 



Delezenne and Dastre and other French observers deny that 

 enterokinase is an enzyme, but consider that it forms a compound 

 with trypsinogen, which is the active proteoclastic ferment trypsin. 

 This view is based upon the observations that a definite amount 

 of enterokinase is required to develop the maximum amount of 



